1. Field of the Invention
The present invention relates generally to apparatus and methods for communicating using spread spectrum techniques, and more particularly for communicating using spread spectrum techniques that conform to IS-95 standards.
2. Discussion of Background Art
Spread-spectrum communication systems currently find widespread use in modern cellular communications devices. Spread spectrum systems allow more users to transmit and receive communications in an ever tighter bandwidth environment.
One technique for spreading a baseband signal so as to fill an entire channel bandwidth is to mix the baseband signal with a Walsh code and a complex pseudo-noise (PN) spreading signal. The Walsh code and PN spreading signal effectively encode the baseband signal by modulating (i.e. chopping) each data symbol within the baseband signal into a number of chips having a chip period (i.e. chip interval) T.sub.c, as is discussed further by Charles E. Cook and Howard S. Marsh, "An introduction to Spread Spectrum," IEEE Communications Magazine, March 1983, and by David P. Whipple, "The CDMA Standard", Applied Microwave & Wireless, Winter 1994, pp. 24-39 (originally published as, "North American Cellular CDMA", Hewlett-Packard Journal, December 1993, pp. 90-97). The complex PN code is given by the following equation: PN(t)=PN.sub.I (t-.delta.)+jPN.sub.J (t-.delta.), where .delta. is a phase offset. Each transmitter within a CDMA network broadcasting over the same frequency spectrum and within a range of a particular receiver is distinguishable by its unique phase offset, .delta.. Each of the transmitters include a number of channels which are encoded and distinguished by different Walsh codes.
Current spread spectrum receivers acquire many different transmitted signals, which, while appearing to be superimposed on one another, are demodulated by correlators that are tuned to accept only transmitted channels corresponding to a particular PN code phase offset and a particular Walsh code. The receiver accomplishes this by stripping away the carrier signal and demodulating the spread spectrum signals with correlators having a matching PN code phase offset and Walsh code.
In order for demodulation to occur successfully, the transmitter's and receiver's PN spreading phase offsets must be synchronized. Delay-locked Loops (DLLs) containing correlators are commonly used to synchronize the receiver's PN code phase offset to the transmitter's PN code phase offset. However, current DLLs contain multiple correlators which add to the expense and complexity of the DLL.
What is needed is an apparatus and method for simplifying the circuitry within delay-locked loops while supporting spread spectrum communications systems conforming to the IS-95 standard.